Z Axis Permittivity Variations in Standard High Speed Multilayer Stackups
Out-of-plane permittivity in glass-reinforced multilayer stackups runs lower than datasheet figures, requiring Z-axis test methods to hold impedance.
Normal Permittivity
Out-of-plane Permittivity
Material properties that describe the dielectric constant measured perpendicular to the plane of the circuit board substrate govern the capacitance and impedance of transmission lines. This normal permittivity dominates the electrical behavior of stripline and microstrip lines where the electric fields are primarily oriented along the vertical axis between the signal trace and the reference plane. This value is distinct from the in-plane permittivity because of the directional fiber orientation in woven glass substrates.
Measurement Technique
Measuring this property accurately is necessary to design transmission lines that meet the target characteristic impedance. The normal permittivity of a laminate is typically measured using a clamped stripline resonator or a split-post dielectric resonator as specified in IPC-TM-650. This measurement is crucial because if the design calculations use the in-plane permittivity instead of the out-of-plane value, the simulated impedance will be higher than the actual impedance on the fabricated board.
The anisotropic nature of the composite resin and glass fibers means that the vertical dielectric constant is lower than the horizontal dielectric constant. This directional difference must be accounted for in 3D electromagnetic field solvers to prevent return loss failures in high-speed digital designs.
Substrate Consistency
Stability in the out-of-plane dielectric constant is critical for maintaining consistent signal propagation delays and impedance profiles across a single board panel. This parameter is verified by fabricators on incoming material lots to ensure compliance with the specification sheet. This test is essential for boards used in phase-critical applications like phased-array radars and high-speed backplanes.